1. Field of the Invention
The present invention relates to a printed circuit board (PCB), and particularly to a PCB with a high density layout of differential pairs and less noise signals.
2. Description of Related Art
Differential pairs are often used in a multi-layer PCB to transmit signals. Referring to FIG. 2, a conventional PCB has edge-coupled differential pairs 17, 18 defined in a signal layer 12, edge-coupled differential pairs 19, 20 defined in a signal layer 13, and edge-coupled differential pairs 21, 22 defined in a signal layer 15 to transmit signals. A reference layer 11 is disposed above the differential pairs 17, 18, a reference layer 14 is disposed below the differential pairs 19, 20, and reference layers 14, 16 are disposed above and below the differential pairs 21, 22. In the PCB of FIG. 2, there are three layers 11, 14, and 16 acting as reference layers, and three layers 12, 13 and 15 acting as signal layers. Because in the conventional PCB, the edge-coupled differential pairs are used to transmit signals, there must be at least one reference layer disposed near the signal layer where the differential pairs are defined to obtain good signal quality, and at least one inner layer of the PCB occupied as the reference layer, therefore the density of the layout of differential pairs is not maximized. Furthermore, noise signals are produced in the differential pairs when two signal layers are too close to each other.
What is needed, therefore, is a PCB with a high density layout of differential pairs and less noise signals.